Accurate characterization of wavefronts produced by an eye is desirable in the field of ophthalmology to facilitate correction of an eye's image-forming system through surgery and/or corrective lens fabrication.
Although various types of aberration measurement apparatus (hereinafter, “aberrometers”) are known, Hartmann-Shack type aberrometers are widely used in commercial ophthalmic applications. FIG. 1 is a simplified schematic illustration of an example of a Hartmann Shack aberrometer 100.
In use, a beam of light from a light source 110 in the aberrometer is directed toward the cornea C of an eye E and onto the retina R by beam splitter 120. The light reflects from the retina and is projected through the cornea, and forms an aberrated wavefront. The aberrated wavefront reenters the aberrometer, and is incident on an array of lenslets 130. The light forms an array of spots d1l-d1n on sensor 140. The locations of the spots relative to the locations that spots would have occupied in the absence of wavefront aberrations provides data that is used to characterize the wavefront and thus detect aberrations. FIG. 2 is a graphical illustration of example intensity values on a representative area of sensor 140 (including a plurality of spots di,j).
A seminal reference in the field of ophthalmic wavefront detection is Liang et al., Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor, Journal of the Optical Society of America, Vol. 11, No. 7, pp. 1-9 (July 1994), the disclosure of which is hereby incorporated by reference in its entirety. Improvements to the technique of Liang et al., id., are taught in Liang and Williams, Aberrations and retinal image quality of the normal human eye, Journal of the Optical Society of America, Vol. 4, No. 11, pp. 2873-2883 (November 1997), and in Williams et al. U.S. Pat. No. 5,777,719, the disclosures of which are hereby incorporated by reference in their entireties.
The ability to accurately measure aberrations and use the measurement information in corrective applications depends on the ability to precisely determine the location of the centers of the spots associated with each lenslet in an array. An inability to accurately detect the centers of all image spots frustrates the characterization of the wave aberrations and subsequent procedures that rely upon those characterizations.
Typically, center coordinates cx, cy of an image spot are calculated by centroid calculation (i.e., summation of weighted values of the incident light intensity I(xi, yi) at points (xi, yi) on sensor 140). Many factors can operate to frustrate accurate centroid determination. One such factor is attenuation of the light as it passes through portions of the patient's eye. The cornea and lens of the eye tend to become more and more opaque with age. The light can also be severely attenuated by the presence of cataracts. FIG. 3 illustrates an exemplary image of aberrometer detector output for a normal eye, while FIG. 4 illustrates an exemplary image of aberrometer detector output for the eye of a cataract patient. As a result of attenuation of light passing through the cataract, image spots in the affected region are very faint and may be barely detectable or completely undetectable. The result of such attenuation is that the center coordinates of the corresponding image spots cannot be determined with desired accuracy, yielding unsatisfactory measurements of aberration in the eyes of afflicted patients.